The Cretaceous period (145 to 66 million years ago) was characterized by a significantly warmer global climate and exceptionally high sea levels. This led to the formation of vast, shallow inland seas, most notably the Western Interior Seaway, which bisected North America. This dynamic marine ecosystem supported a wide array of life. While massive marine reptiles like mosasaurs and plesiosaurs often capture the imagination, the true ecological foundation and most diverse vertebrate group was the fish population. Fish dominated all trophic levels, from the smallest plankton-feeders to the largest apex predators.
The Reign of the Teleosts
The Cretaceous marked a profound evolutionary watershed for the Teleostei, the infraclass that includes nearly all modern bony fish. This period saw explosive diversification and the establishment of anatomical traits that gave them advantages over older, more primitive fish groups. Their success stemmed from a trend toward lightweight, fully ossified skeletons, which improved agility and swimming efficiency.
A major innovation was the development of a fully symmetrical, or homocercal, caudal fin. In this structure, the upper and lower lobes are equal in size, providing superior thrust and a more streamlined body compared to the asymmetrical tails of ancient fish. Teleosts also evolved a highly mobile premaxilla in the jaw, allowing them to rapidly protrude their mouths to suck in prey. This specialized jaw function broadened their dietary range and hunting effectiveness. This combination of skeletal lightness, powerful propulsion, and specialized jaws allowed teleosts to dominate virtually every aquatic niche in the Cretaceous seas.
Apex Predators of the Ancient Seas
The oceans of the Cretaceous featured fearsome fish that stood as impressive counterparts to the famous marine reptiles. Among the most formidable was Xiphactinus audax, often nicknamed the “giant predatory tarpon.” This bony fish grew to lengths of four to five meters, and its long, dagger-like teeth made it a terrifying predator in the Western Interior Seaway.
Fossil evidence, including a famous specimen found with a nearly intact four-meter-long fish in its stomach, suggests Xiphactinus was an opportunistic and aggressive hunter that swallowed prey whole. It specialized in fast-swimming prey like smaller fish and cephalopods. Although Xiphactinus was a massive apex predator, it was sometimes preyed upon by larger marine reptiles like mosasaurs and plesiosaurs. The existence of these massive bony fish alongside marine reptiles confirms that fish maintained their position as top predators in specific ecological niches.
The Cartilaginous Fish Sharks and Rays
Separate from the bony fish, the Chondrichthyes (sharks, rays, and chimaeras) also underwent significant evolutionary radiation during the Cretaceous. This period saw the rise of many modern groups, including the ancestors of the Lamniformes (mackerel sharks). These new forms had more flexible skeletons and efficient predatory jaws, allowing them to compete effectively.
One widespread Cretaceous shark was Squalicorax, the crow shark, a Lamniform reaching up to 4.8 meters. Its serrated teeth resembled those of a modern tiger shark, indicating a versatile diet. Squalicorax were effective predators of fish and prominent scavengers; their teeth marks are found on the bones of marine reptiles and terrestrial dinosaurs. Meanwhile, the Batoidea (rays and skates) also diversified, evolving flattened body plans and specialized dental plates adapted for crushing hard-shelled prey like mollusks and crustaceans on the seafloor.
Surviving the Great Extinction
The cataclysmic impact event at the end of the Cretaceous period profoundly reshaped life on Earth, but fish proved remarkably resilient compared to many other vertebrate groups. While marine reptiles like mosasaurs and plesiosaurs were completely wiped out, the Teleostei and Chondrichthyes suffered less severe losses. Extinction rates for teleosts were still high, with estimates suggesting that between 25 and 60 percent of genera disappeared.
Many surviving fish groups had generalist diets, allowing them to cope with the collapse of the food chain after the impact decimated plankton. Survivors also included groups that sought refuge in deeper waters or less-affected freshwater environments, shielding them from surface devastation. This selective survival cleared numerous ecological niches, allowing the surviving teleost lineages, such as those leading to modern cod, eels, and perch-like fish, to undergo explosive diversification in the subsequent Paleogene period, establishing the foundation for modern fish dominance.